(1) Field of the Invention:
The present invention relates to a process for preparing a sealed laminated vessel. More particularly, the present invention relates to a laminated vessel in which an excellent seal is formed between the open end of the barrel portion of the vessel and a lid member by heat sealing and mechanical lap-seaming notwithstanding the presence of a seam step.
(2) Description of the Prior Art:
A laminated sheet which comprises a paper substrate, a metal foil and inner and outer surface layers of a heat-sealable resin has been widely used as a material having excellent storage property. When this laminated sheet is formed into a sealed vessel, the end edges of the laminated sheet are piled together and heat-sealed to form a cylindrical or tapered barrel portion having a seam and a lid member is attached to at least one open end of the barrel portion by heat sealing.
However, a sealed vessel having this heat-sealed structure is defective in that it is often difficult to securely bond the lid to the barrel portion. More specifically, in the first place, in case of a vessel of a rigid material having a processability, such as a metal can, secure sealing can be attained by attaching the peripheral edge portion of a lid to the flange of a can barrel through a sealing compound and double-seaming the flange and the peripheral portion of the can lid, but in case of a laminated sheet including paper, the double-seaming operation is impossible and a seal is formed only by heat sealing. Accordingly, various problems described below arise in case of a laminated sheet including paper. When the heat sealing operation is performed, if the portion to be bonded is flat and two-dimensional as in case of sealing of ends of a bag-like vessel or formation of a side seam in a cylindrical vessel, the operation is relatively easy and a seal having a high reliability can be obtained, but if the portion to be bonded is three-dimensional and has no sufficient area as in case of bonding of the peripheral edge portion of a lid to the open end edge of a barrel portion, the heat sealing operation is not always easy and it is difficult to securely form a seal having a high reliability along the entire circumference of the heat-sealed portion.
In the second place, a very large step is formed between a seam formed by lap bonding and a portion other than the seam in the open edge portion, to be heat-sealed, of the barrel, and leakage is readily caused in this step portion. If the thickness of the barrel wall of the vessel is d; a step having a height of 2d is formed when a double-wall structure is formed in the end edge portion, and a step having a height of 3d is formed when a triple-wall structure is formed in the end edge portion. Thus, a very large step is formed between the seam and the portion other than the seam. Under ordinary heat sealing conditions, it is difficult to completely fill this step portion with a heat-sealable resin between the lid member and the end edge of the barrel portion, and it is substantially impossible to completely avoid leakage.
In case of a composite vessel formed by heat-sealing a laminate comprising a paper substrate, adsorption water in the paper substrate is evaporated by heating conducted at the step of heat-sealing the resin layer to form bubbles in the resin layer to be heat-sealed, and the sealing property of the heat-sealed portion is further reduced.
Moreover, in this composite vessel, if the cut edge of the laminate is exposed to the open end of the barrel portion, permeation of water or water vapor is caused through the exposed portion of the paper substrate, resulting in reduction of the shape-retaining property, mechanical strength and content storage property of the vessel.